Is the Tunisian colloquial saying "أنجّم" (anjjim) meaning "I can" forbidden, based on the fatwa that "تنجيم" (tanjim - astrology/fortune-telling) is forbidden?

The key lies in meanings and intentions. Imam Bukhari titled a chapter: "If they said: 'We have apostatized (saba'na),' but they did not say 'We have submitted (aslamna)' well." The purpose of this title is that intentions are considered based on their proofs. There is no harm in the customary uses of words unless they are specifically forbidden for their inherent meaning. The word "najm" (star) is not restricted to celestial bodies and looking at them in a forbidden manner. Therefore, there is no harm in using it with the aforementioned meaning, and the statement of anyone who claims its prohibition has no basis.